Fluorosis is endemic in at least 25 countries globally; more than 200 million people are exposed to high concentrations of naturally occurring fluoride (F?). Risk areas are mostly located in arid and semi-arid tropical and volcanic regions such as in the Main Ethiopian Rift Valley (MER), where a large fraction of groundwater sources have elevated concentrations of F?, affecting the health of nearly 8.5 million people. Drinking groundwater has traditionally been considered the major route for F? exposure in this population; however, our preliminary measures of urinary F? and other recent studies suggest that dietary sources may contribute to exposure. We expect that total F? exposure from these sources can be more reliably estimated through nail F? analysis, which will also enhance prediction of health outcomes. In addition, while many epidemiological and clinical studies have shown a strong correlation between F? exposure and dental fluorosis (DF), skeletal fluorosis (SF) is more complex and variable, and remains poorly understood. Fluoride exposure in the presence of sufficient calcium (Ca) leads to an osteosclerotic form of SF, whereas in nutrition-deficient individuals, SF manifests as bone resorption, particularly among children. This novel and highly innovative project will employ clinical/radiographic fluorosis examinations and, for the first time, apply Ca isotope techniques developed in the field of geochemistry to detect F?-induced metabolic bone disorder. During the mentored phase, under the combined mentorship of highly qualified experts in environmental geochemistry and health (primary mentors), skeletal fluorosis, F? metabolism and toxicity (co-mentors), and bone biology, toxicology, biostatistics, epidemiology and nutrition (collaborators), Dr. Tewodros R. Godebo, the candidate, will accomplish two key goals. First, he will improve understanding of all sources of F? exposure through measurement of F? in individuals? fingernails and link this to DF outcomes (Aim 1). Second, he will obtain training in clinical diagnostic skills on SF and laboratory analytical methods, and additional didactic courses in biostatistics, epidemiology and nutrition, which will lay the foundation for research to be undertaken in the independent phase of this award (Aims 2-3). In this second phase, the candidate will build on his postdoctoral work to explore the effect of F? exposure on SF using clinical and radiographic methods (Aim 2). He will also explore the effect of over-exposure to F? on natural (biologically-induced) Ca isotope variations in urine (Aim 3). At the completion of the K99/R00 project, the project will have: 1) offered new insights on the effect of F? exposure in the pathogenesis of SF (and DF); 2) for the first time, revealed the role of a novel Ca isotope biomarker application in urine for monitoring the pathogenesis of SF; 3) helped the candidate to achieve his long-term career goal of becoming an established scientist working independently in an academic institution; and 4) positioned the candidate to write and submit future R01 grants and collaborate with peers from various disciplines, building on the didactic and methodological knowledge gained in this research.